Exemplary embodiments of the present invention relate to a fuel cell system with at least one fuel cell and the use of such a fuel cell system.
Fuel cell systems are known in themselves from the general prior art. A process air flow is thereby frequently made available to the fuel cell via an air conveying means. The process air flow flows through a cathode region of the fuel cell to supply it with atmospheric oxygen. The air fed to the fuel cell is compressed through the air conveying means, for example a flow compressor, a screw compressor, a Roots blower or similar, and is thereby correspondingly heated. When using a PEM fuel cell in the fuel cell system, this heating of the process air can contribute to a rapid drying out of the relatively sensitive membranes in the fuel cell. A heat exchanger is therefore frequently provided in such fuel cell systems as a charge air cooler, through which the process air flows after the air conveying means and through which a cooling medium, for example the comparatively cool exhaust air from the region of the fuel cell, flows in order to cool the process air flowing to the fuel cell after the air conveying means and thus to increase the lifespan and performance capability of the fuel cell.
It is additionally known from the general prior art that the cold start capability plays an important role for fuel cell systems and here in particular for fuel cell systems used in motor vehicles. Considerable resources are used in order to be able to start the fuel cell systems safely, reliably, and in particular very rapidly even at temperatures below freezing point. German Patent Application DE 10 2008 047 871 A1 discloses bringing about rapid heating of a fuel cell stack by means of various heat sources in a cooling circuit of the fuel cell stack. Should all these measures not suffice, the aforementioned patent application proposes conveying hydrogen to the cathode side of the stack and burning this together with the process air in the region of the electrocatalysts present in the fuel cell. A comparatively high heat energy can thus be produced in the region of the fuel cell itself.
This measure with the introduction of a comparatively large quantity of hydrogen into the cathode region of the fuel cell has the serious disadvantage that by supplying the hydrogen into the cathode region the catalyst present there is used for combustion, thus for the thermal conversion of the hydrogen with the oxygen of the process air. This results in a comparatively high point-by-point development of heat, which massively impairs the lifespan of the fuel cell.
Exemplary embodiments of the present invention provide a fuel cell system that guarantees very rapid heating of the fuel cell system without having to take into account the abovementioned disadvantages in relation to the lifespan of the fuel cell.
According to the invention a fuel cell system with a heat exchanger is used, which is used as a charge air cooler for cooling the process air compressed by the air conveying means and thereby heated. According to the invention a region with a catalytically active material is thereby arranged before or in the region of the heat exchanger, through which the process air flow flows. In addition fuel is fed, as required, to the region with the catalytically active material. This results in the process air conveyed to the fuel cell not only being able to be cooled as in conventional operation but also as required, and here in particular in case of a cold start, being correspondingly heated through the addition of fuel into the region with the catalytically active material through a catalytic conversion of oxygen and fuel and in case of hydrogen-containing fuel being able to be moistened through the product water produced. This results in a very simple and efficient possibility of realizing through the supply of fuel a heating of the process air and thus of the fuel cell and the fuel cell system. As a result of the catalyst designed specially for this purpose and arranged before or in the region of the heat exchanger the heat is not produced in the region of the fuel cell itself but instead outside thereof. It is then introduced by the process air flow into the region of the fuel cell. Disadvantages relating to the lifespan of the fuel cell, as are known and usual in the structures according to the prior art, can thereby be prevented as in the region of the fuel cell itself no fuel reaches the cathode region thereof and must be correspondingly converted. The fuel cell system itself can thus be brought very quickly to operating temperature, in particular having regard to all components arranged in the region of the air supply.
In principle any type of cooling medium can thereby flow through the heat exchanger used as a charge air cooler on the cooling side. This heat exchanger can be designed, for example, integrated in a cooling circuit so that the process air heated after the air conveying means is cooled by the cooling medium of a cooling circuit which can, for example, also cool the fuel cell. In this case the heat produced through the catalytic conversion of the fuel with the oxygen, before or in the heat exchanger, can heat through this not only the process air flow but also the cooling medium flow so that this heats the cooling circuit and additionally also contributes to a more rapid heating of the fuel cell system.
In an advantageous embodiment of the fuel cell system according to the invention the region with the catalytically active material is integrated into the heat exchanger. This integration of the catalytically active material onto the side of the heat exchanger through which the process air flow flows thereby facilitates a very compact structure as the component of the heat exchanger serves both as a charge air cooler and also—if required—for the catalytic conversion of substances on its side through which the process air flows.
In a preferred development of the fuel cell system according to the invention the heat exchanger is coated in full or in part with the catalytically active material in the region through which the process air flow flows. Such a coating of the heat exchanger with the catalytically active material is comparatively easily to achieve. Therefore, with minimum use of catalytically active material, for example platinum or palladium, a correspondingly efficient catalytic conversion of air and fuel can be achieved. Unlike with the also conceivable pouring of pellets provided with catalytically active material or similar, the introduction of impurities into the region of the fuel cell is virtually excluded, meaning that filter elements and similar that would cause an unnecessary pressure loss are not necessary.
In a very favorable and advantageous development of the fuel cell system according to the invention the cooling medium flow is an exhaust air flow from a cathode region of the fuel cell.
This preferred structure uses the comparatively cool exhaust air from the cathode region of the fuel cell in regular operation to cool the process air for the fuel cell heated after the air conveying means. Because under correspondingly cold start conditions the compression of the process air does not suffice in order to heat this correspondingly significantly in order to thus prevent condensation of liquid and/or freezing of this liquid, according to the invention the region with the catalytically active material is provided before or in the region of the heat exchanger in order to heat the process air flow to the fuel cell through the addition of a fuel. This heat now also benefits the exhaust air flow, which is also still comparatively cold in such an operating situation. The exhaust air flow can thus be correspondingly heated and freezing of the exhaust air pipe or for example a turbine arranged in the exhaust air flow through condensing and freezing droplets can be prevented. The introduction of heat during the cold start of the fuel cell system through the catalytically active material in the region of the heat exchanger thus also serves to guarantee the operating capability of the fuel cell system on the exhaust air side in an ideal way.
The fuel cell system according to the invention has the advantage that it has a very compact and simple structure and provides a fuel cell system that can be started very simply and efficiently even in case of very low ambient temperatures. The preferred use of such a fuel cell system is therefore for the provision of electrical power in a transport means, which together with the fuel cell system, forms a mobile system. In such mobile systems a start from adverse conditions, in particular from temperatures below freezing point, can frequently occur. A rapid start of the system is indispensable especially in such systems as this is expected by the user, for example, when used in a motor vehicle as a transport means. Indeed, in motor vehicles with conventional drive systems a comparable behavior is present and is known and usual. The fuel cell system according to the invention can therefore fully exhibit its advantages in particular in such a structure.